All types of tea – green, black and oolong – are produced from the Camellia sinensis plant using different methods. Fresh leaves from the plant are steamed to produce green tea, while the leaves of black tea and oolong involve fermentation.

The evidence on green tea

Does drinking green tea protect you from cancer?

There is no evidence drinking green tea protects against different types of cancer. A review from 2009 involving 51 studies, with more than 1.6 million participants, looked for an association between drinking green tea and cancers of the bowel, prostate, breast, mouth and lungs. The authors of the review concluded evidence of a link between green tea and cancer was weak and "highly contradictory".

A more recent 2015 study looked at the cancer-fighting effects of a compound found in green tea when combined with a drug called Herceptin, which is used in the treatment of stomach and breast cancer. Initial results in the laboratory were promising and human trials are now being planned.

But this shouldn't be taken as official advice that drinking green tea while taking Herceptin will make it more effective.

Can green tea aid weight loss?

It's thought the antioxidants catechin and caffeine found in green tea may have a role in helping the body burn more calories – sometimes referred to as speeding up the metabolism – which can help weight loss.

Green tea preparations used for losing weight are extracts of green tea that contain a higher concentration of catechins and caffeine than the typical green tea beverage prepared from a tea bag and boiling water.

A well-conducted review from 2012 of 18 studies involving 1,945 people found no significant effect of weight loss from drinking green tea.

Does green tea cut cholesterol?

A good-quality review from 2013 of 11 studies involving 821 people found daily consumption of green and black tea (as a drink or a capsule) could help lower cholesterol and blood pressure thanks to tea and its catechins. The authors of the review caution that most of the trials were short term and more good quality long-term trials are needed to back up their findings.

Another good-quality review from 2011 found drinking green tea enriched with catechins led to a small reduction in cholesterol, a main cause of heart disease and stroke. However, it's still not clear from the evidence how much green tea we'd need to drink to see a positive effect on our health, or what the long-term effects of drinking green tea are on our overall health.

Can green tea help prevent or delay Alzheimer's disease?

Evidence of a positive link between drinking green tea and Alzheimer's disease is weak. A 2010 laboratory study using animal cells found a green tea preparation rich in antioxidants protected against the nerve cell death associated with dementia and Alzheimer's disease.

Whether these lab results can be reproduced in human trials remains to be seen. As such, the findings do not conclusively show green tea combats Alzheimer's disease.

Can green tea lower blood pressure?

A 2014 survey of data from previously published studies looked at the evidence of whether drinking green tea could help lower blood pressure. There was evidence of a modest reduction in people with high blood pressure who consumed green tea. But whether this reduction would lead to clinically significant results, such as preventing the onset of heart disease or stroke, is unclear.

Can green tea prevent tooth decay?

A small study from 2014 looked at how effective a green tea mouthwash was in preventing tooth decay compared with the more commonly used antibacterial mouthwash chlorhexidine. The results suggested they were equally effective, though green tea mouthwash has the added practical advantage of being cheaper.

The dietitian's verdict on green tea

She says: "In the Far East, green tea has been used as a treatment for a variety of conditions ranging from arthritis to weight loss, as well as a preventative measure for diseases such as cancer, although the evidence for the majority of these conditions is weak or lacking.

"However, as a social drink, it appears to be safe in moderate amounts, so lovers of green tea can continue to enjoy it."

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